The present invention is directed to a method and apparatus for stopping an elevator moving along a controlled path, exactly at the desired point. The elevator is provided with a stopping brake and the above is carried out by controlling the time of commencement of braking.
The present invention concerns a procedure for stopping exactly at the desired point a means moving along a controlled path and provided with stopping brake, such as a lift, by controlling the time of commencement of braking.
The accuracy with which the lift stops at the storey floor level is one of the essential problems in lift technology and is receiving increasing attention. Lift use is indispensable for instance to a handicapped wheelchair patient; it is required in such cases that the stopping accuracy of the lift allows unimpeded passage into and out of the lift cage. It is also required with increasing frequency that slow and simple residential building lifts meet these requirements of accurate stopping. An acceptable stopping accuracy is about .+-.15 to .+-.20 mm.
The stopping accuracy of the lift cage is mainly dependent on the characteristics of the drive system driving the lift. In fast passenger lifts (over 1.0-1.5 m per second) a feedback-connected control system is commonly used, which endows the lift with good running characteristics and with good stopping accuracy as well. In slower lifts (v.ltoreq.1.0 m per second), the commonest drive system is a squirrel cage motor drive with either one or two speeds. The single-speed squirrel cage motor is the simplest and cheapest drive system, but its limitations are met in the accuracy of stopping, which is about .+-.70 mm when the nominal speed is 0.63 m/s. Since residential buildings constitute the main area where single-speed lifts are employed, it is consequently important that the stopping accuracy can be improved with a view to facilitating the lift travelling of aged and handicapped persons. The stopping accuracy of the single-speed lift has been improved by a procedure, where errors in stopping accuracy are caused by changes in the torque properties of the lift's brake.
It is possible with the so-called two-speed drive system to achieve the above-mentioned stopping accuracy of .+-.15 to .+-.20 mm. In such case the velocity of the lift cage is reduced before the storey floor level, to 1/4 or 1/6 of the nominal speed, and final arrest is accomplished out of this lowered speed. However, the two-speed drive system has the drawback that the initial cost of the lift increases and furthermore, that replacement of single-speed lifts already in use by two-speed lifts is an expensive undertaking.